The development of high-efficiency and stable electrocatalysts is significant for energy conversion and storage. The oxygen evolution reaction (OER), a pivotal half reaction, is seriously limited in its practical applications due to its sluggish kinetics and thus an excellent electrocatalyst for OER is urgently required. In this paper, we design a novel Co3O4 nanomesh (Co3O4 NMs) with high density grain boundaries (GBs), which functions as a highly efficient and steady OER electrocatalyst. The optimal Co3O4 NMs-500 can achieve a low overpotential of 295 mV at a current density of 10 mA cm-2, and a small Tafel slope of 31 mV dec-1, which exceeds the commercial Ir/C, as well as the majority of other catalysts reported in the literature. The Co3O4 NMs-500 also exhibit promising durability, with a negligible decline in activity after 18 h of operation. Detailed studies indicate that the presence of GBs leads to more exposed active sites and the enhanced adsorption of intermediate species on Co3O4 NMs-500, thereby improving the OER's catalytic activity. This work not only relates to the activity-GBs relationship, but also opens up a unique perspective for the design of the next generation of electrocatalysts.